feat: add fibonacci function with edge case handling

[fletc89-cyber]

Summary

Implements fibonacci(n) in src/math_utils.py with edge-case handling, addressing issue #1 (bounty-plaza#976).

Changes

• src/math_utils.py: Added fibonacci(n) function

  • Returns the nth Fibonacci number (0-indexed)
  • fibonacci(0) -> 0, fibonacci(1) -> 1, fibonacci(5) -> 5, fibonacci(10) -> 55
  • Raises ValueError("n must be non-negative") for n < 0
  • Iterative implementation (O(n) time, O(1) space)

• tests/test_math_utils.py: Added comprehensive tests

  • Base cases (0, 1)
  • Normal cases (2, 3, 5, 10)
  • Negative edge case (raises ValueError with correct message)
  • Large input (fibonacci(20) == 6765)

Test Results

All 8 tests pass:

tests/test_math_utils.py::test_add PASSED
tests/test_math_utils.py::test_multiply PASSED
tests/test_math_utils.py::test_fibonacci_base_cases PASSED
tests/test_math_utils.py::test_fibonacci_normal_cases PASSED
tests/test_math_utils.py::test_fibonacci_negative_edge_case PASSED
tests/test_math_utils.py::test_fibonacci_large_input PASSED
tests/test_string_utils.py::test_reverse_string PASSED
tests/test_string_utils.py::test_to_uppercase PASSED

Bounty Reference

• Bounty issue: [Bounty] Add fibonacci function with edge case handling zhangjiayang6835-cyber/bounty-plaza#976
• Source issue: Add fibonacci function with edge case handling #1

Closes #1

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This PR was created by an AI agent (OpenHands) on behalf of fletc89-cyber.